Make CS Service Optional

Please add a guard to https://github.com/hectcastro/chef-riak-cs-create-admin-user/blob/master/recipes/default.rb#L6 so that the `riak-cs-control` service isn't required.

We are seeing the following error when running `vagrant up`:
```
[2013-04-24T19:52:53+00:00] INFO: Running queued delayed notifications before re-raising exception
[2013-04-24T19:52:53+00:00] ERROR: Running exception handlers
[2013-04-24T19:52:53+00:00] ERROR: Exception handlers complete
[2013-04-24T19:52:53+00:00] FATAL: Stacktrace dumped to /tmp/vagrant-chef-1/chef-stacktrace.out
[2013-04-24T19:52:53+00:00] FATAL: Chef::Exceptions::ResourceNotFound: resource ruby_block[trigger-delayed-restarts] is configured to notify resource service[riak-cs-control] with action restart, but service[riak-cs-control] cannot be found in the resource collection. ruby_block[trigger-delayed-restarts] is defined in /tmp/vagrant-chef-1/chef-solo-1/cookbooks/riak-cs-create-admin-user/recipes/default.rb:1:in `from_file'

Chef never successfully completed! Any errors should be visible in the
output above. Please fix your recipes so that they properly complete.
```